Individual devices are conventionally isolated on a semiconductor substrate using a technique termed LOCal Oxidation of Silicon (LOCOS). LOCOS comprises forming a thin pad oxide layer on an underlying substrate. A blanket silicon nitride layer is then formed on the thin pad oxide layer. Silicon nitride is very slow to oxidize. Thus, if a sufficiently thick layer of silicon nitride is used as an oxidation mask, the underlying substrate is protected from oxidation. The thin pad oxide layer serves to prevent introduction of crystallographic dislocations in the underlying substrate, caused by lattice strain between silicon and silicon nitride. Active regions are then defined in the blanket silicon nitride layer by masking and etching to remove the silicon nitride in field regions. Active regions remain covered by silicon nitride, preventing subsequent oxidation of the underlying substrate. The field regions are then oxidized using an oxidizing ambient to form field oxide. Subsequent to LOCOS isolation, devices are formed in the active regions.
Conventionally, oxidation in a LOCOS process comprises utilizing an oxidizing ambient at atmospheric pressure. However, field oxide formed using atmospheric LOCOS techniques is plagued by lateral encroachment of oxide under the silicon nitride mask layer, forming a "bird's beak." Lateral encroachment is undesirable because it severely decreases semiconductor chip density. In order to fulfill consumer desires for faster microprocessors and other semiconductor chips, chip density must be maximized. Thus, the "bird's beak" problem prevents semiconductor devices from achieving their optimal electrical performance.
Many ways of decreasing such encroachment have been studied. One particularly effective method of alleviating the "bird's beak" phenomenon is by oxidizing the underlying substrate using pressures higher than conventionally-used atmospheric pressure. Oxide formed using high pressures is also less prone to crystal defects throughout the oxide layer. Crystal defects are undesirable because they allow diffusion through the oxide layer, potentially resulting in leakage currents that are detrimental to optimal device performance. For example, refresh times for dynamic random access memory arrays are optimized by decreasing leakage current from memory cells. Optimized refresh times are critical for faster electrical performance and lower power consumption. Power consumption is minimized because the memory cell array does not need to be refreshed as often when the leakage current is minimized.
While oxide formed using high pressure is relatively free of crystal defects, the oxide thinning effect prevents the effective use of oxide formed using high pressure oxide, particularly in narrow pitch structures. The oxide thinning effect results in increased leakage current when thinning is severe. For illustration, given 0.6 .mu.m pitch field oxide regions produced by a 0.25 .mu.m process, the thickness of the field oxide is approximately 2,500 angstroms in the center of the field oxide regions, but only approximately 1750 angstroms at the edges of the field oxide regions. This 70% thickness undesirably increases the leakage current through such field oxide regions. Narrow pitch structures are common in semiconductor integrated circuits, particularly memory and logic circuitry. Thus, oxide formed using high pressure has not been effectively used in such applications.
Another problem experienced with the use of high pressures for forming field oxide is that surface contamination often causes "hot spots" in the field oxide layer at the interface with the substrate. Contaminants, such as sodium, potassium, and fluorine are often undesirably introduced during the semiconductor fabrication process, particularly from deposition chambers and furnaces. When such contaminants are present on a substrate during high pressure oxidation, they act as nucleation sites. Excess oxide growth occurs at such nucleation sites. High pressure oxidation significantly magnifies this problem. Excess growth has been known to cause field oxide regions on a wafer that are approximately 30 to 40% thicker than the rest of the field oxide. On large wafers, such excessive growth regions have been reported covering areas as large as a dime, and even a quarter. This is extremely undesirable because it causes problems with back end of line (BEOL) technology, particularly presenting step coverage problems for interconnect and metal line fabrication, severely decreasing yields, which increases fabrication cost.
There is a need for a method for forming field oxide having a low crystal defect density. There is a further need for a method for forming field oxide that is not adversely affected by the oxide thinning effect. A method to alleviate the "hot spot" effect seen when oxidizing substrates at high pressures is needed in order to take advantage of the advantageous effects of high pressure oxidation. As device density is becoming increasingly important and cost pressures increase, it is critical to provide isolation techniques having minimal leakage current and providing high yield.